Mortality in AIDS patients is increased by the erosion of whole-body nitrogen and lean body mass. This cachexia is not prevented by available therapeutic modalities of nutritional and metabolic support, and its pathogenesis is enigmatic. Most of the nitrogen excreted during cachexia is converted into urea from amino acids derived from the protein-metabolic precursor pool. TNF~ and other cytokines have been implicated in the mediation of HIV-associated cachexia because they cause whole-body nitrogen depletion and enhanced rates of ureagenesis in vivo. The molecular basis of cytokine-induced nitrogen loss is mysterious however, because cytokines do not act directly on skeletal muscle to cause protein breakdown in vitro. We have recently developed a hypothesis of cytokine-induced whole-body nitrogen loss that reconciles the discrepancy between the ability of cytokines to cause net nitrogen loss in vivo, but not in cultured myocytes. This hypothesis proposes that cells of the immune system, particularly macrophage/monocytes, contribute directly to the whole-body nitrogen losses by metabolizing arginine into urea. This macrophage pathway, which may account for the daily excretion of more than 1.25 g urea, equivalent to 14.5 g wet muscle mass, is capable of actively draining arginine from the protein-metabolic precursor pool. Preliminary experiments reveal that cytokine-stimulated macrophages rely on enhanced arginine transport to produce urea. In addition, HIV infected monocytes are stimulated by TNF to produce urea. These observations led to the rational design and development of "tetrahydrazone-14," a guanylhydrazone compound that inhibits monocyte ureagenesis and prevents increased whole-body urea losses in an animal model of cachexia. The objectives of the studies outlined herein are to address this hypothesis in the pathogenesis of AIDS cachexia by characterizing urea synthesis in HIV-infected macrophages in vitro; by measuring the contribution of macrophage ureagenesis to whole-body nitrogen losses in AIDS patients; and by investigating the effects of inhibiting macrophage ureagenesis in an animal model of TNF-associated cachexia. The experimental strategy proposed will accrue data from parallel studies in tissue culture systems, animal models, and patients, and will utilize the urea synthesis inhibitor tetrahydrazone-14 to identify the relevant regulatory mechanisms. The significance of characterizing this amino acid metabolic pathway in immunological cells lies in advancing our understanding of the molecular basis of whole-body nitrogen loss in cachexia. This improved understanding is expected to guide the rational development of future therapeutics and diagnostics for the metabolic and nutritional management of AIDS.